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Jean-Louis BRIAUD Texas A&M University
DESIGNING FOR DOWNDRAG ON UNCOATED AND
BITUMEN COATED PILES
Jean-Louis BRIAUD, PhD, PE
President of ISSMGEProfessor and Holder of the Buchanan Chair
Texas A&M University
Piling and Deep Foundations Middle East 2010
1 February 2010
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1. What is downdrag?
2. Predicting pile behavior subjected to downdrag
3. Design and selection of bitumen
4. Example problem
5. Full scale field test in New Orleans, LA, USA
6. Full scale field test in Edmonton, AL, Canada
7. Conclusions
Content Outline
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What is downdrag?
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When to Design for Downdrag
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Design to Avoid these Three Conditions
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Static Equilibrium
max t n p pQ Q F F Q
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Finding the Depth of Neutral Point
pile(NP) = soil(NP)
soil(NP)= read on
consolidation
settlement profile (given)
pile(NP) = point + elastic
point = soil (@Z=L) +punch
elastic = elastic compression
from bottom to NP
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Pile Point Behavior
2
(1 )4
p
punch
s
Q D
v AE
punch = Pile point movement
= Poissons ratio
Qp = Point resistance
A = Area of pile point
D = Diameter of pile point
Es = Soil modulus
For clays
Es = 100 Su = EpmtFor sands
Es (kPa) = 800 N = 2 Epmt
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Pile Point Behavior
12
elastic p pL ZQ F
AE
elastic
= Elastic compression
of pile
Qp = Point resistance
Fp = Positive friction force
L = Depth of pile point
Z = Location of NPA = Area of pile point
E = Pile modulus
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FOR PILES IN CLAY (short term and long term)
fmax = Su fmax = ov
Maximum Friction
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FOR PILES IN SAND fmax = ov
fmax (kPa) = 5 (N)0.7
N = SPT blow count
Maximum Friction
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FOR PILES IN SANDS AND IN CLAYS
Clay (short term) qmax = 9 Su
Clay (long term) qmax = ov Nq
Sand (short & long term) qmax (kPa) = 1000 (N)0.5
Sand (short & long term) qmax = ov Nq
Maximum Point Resistance
2 max
max(1 )
4s
q Dv
AE
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Downdrag on Pile Groups
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Downdrag on Pile Groups
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Reduction
Factors for
Downdrag in PileGroups
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- Developed at Texas A&M University
- Analyzes axially-loaded single piles under both
positive and negative skin friction- Bitumen coated piles can also be analyzed with
the program
PILENEG
http://ceprofs.tamu.edu/briaud/pileneg/pileneg.zip
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PILENEG Inputs- Top load on pile
- Pile data (cross section area of the pile, pile
perimeter, embedded pile length, pile modulus)- Soil data (profile of maximum friction, soil
settlement profile, soil modulus and Poissons
ratio at point, ultimate bearing pressure at point)
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- Top load, top settlement, depth of bitumen
coating, depth to neutral point, maximum load,
maximum stress and pile point load
PILENEG Outputs
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WHAT IS BITUMEN?
A black viscous substance. The residue left at theend of the refining process of crude oil, made of
high molecular weight hydrocarbons
WHAT IS A PRIMER?
A liquid made of half bitumen and half solvent to
liquefy the bitumen for easy application and filling
the holes. The layer left is largely the bitumen
after drying.
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Bitumen is a viscous material = (d/dt)
Behavior of Bitumen
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= (d/dt)
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= (d/dt)
decreases when d/dt increases
d/dt x 10 leads to ~ / 2
decreases when T increases
T x 2 leads to ~ / 10
much more sensitive to T than d/dt
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= (d/dt)
Storing
Driving
Downdrag
Particle penetration
SOIL
Bitumen Selection
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Storage Criterion
2
( )req storage gtdh
= Bitumen mass density
g = Acceleration due togravity
t = Max anticipated storage
period
d = Bitumen coatingthickness
h = Allowable bitumen flow
distance (usually taken as
equal to d)
Storage requires:
> req(storage)
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Driving Criterion
2
( , )
a a
req driving air
a
v d
h
= Bitumen mass density
va = Velocity of bitumen in
the air (taken as the velocity
of the hammer at time of
impact)
d = Bitumen coating thickness
h = Allowable bitumen flow
distance (usually taken asequal to d)
fmax = Maximum friction
t = Max anticipated storage
period
Driving in air requires:
> req(driving, air)
max
( , )
s
req driving soil
f td
h
Driving in soil requires: > req(driving, soil)
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Downdrag Criterion
( )
2 1
2 1
bit bit bit
req downdrag
s
d d
s s st t
bit = Allowable shear stress
on bitumen (bit ~ 0.1 s,avg)s,avg = Soil shear strength
d = Bitumen coating thickness
s = Settlement rate
s1 , s2 = Settlements of theground surface at times t1and t2, respectively (after
placement of an embankment
or other load)
Downdrag requires:
< req(driving)
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Particle Penetration Criterion
' '
H OVK
FOR DRIVEN PILES: K = 1
FOR BORED PILES: K = K0 = (1-sin)OCR0.5
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Bitumen Design Criteria Summary
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EXAMPLE:
Find allowable top load fora top settlement < 14 mm
1) Pile Ult. Capacity
Qu = (25x1.2x30) + 1000
Qu = 900 + 1000 = 1900 kN
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EXAMPLE:2) Assume a top load = 500 kN
and find the maximum depth toNP
500 + X = 1000 + (900 - X)
X = 700 kN or 23.3 m of friction
NP(soil)
= 35 mm (settlement profile)
top = 35+(850*23300)/(0.09*2E7) = 46 mm
(This is more than the allowable settlement)
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EXAMPLE:3) Assume a top load = 100 kN
and NP = 29 m
Qp = 100 + 870 - 30 = 940 kN
p = 4.7 mm
NP(pile) = 4.7 + (955*1000)/(0.09*2E7)
= 5.2 mm ~ NP(soil)
top = 5 + (535*29000)/(0.09*2E7)
= 13.6 mm
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EXAMPLE:4) Use bitumen coating with shear
strength = 2.5 kN/m2 and try a topload of 500 kN (Assume NP = 29 m)
NP(soil) = 5 mm
Qp = 500 + 87 30 = 557 kNp = 2.8 mm
top
= 5 + (543.5*29)/(0.09*2E7)*(1000)
= 13.8 mm
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top = 46 mm top = 14 mm top = 14 mm
with bitumen
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Full Scale Field
Test:
Alberta,Edmonton, Canada
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CONSTRUCTION AND
SPECIFICATIONS
Clean surfaces (free of dust and grease)
Apply the primer (wait 24 hrs for drying) Apply the bitumen (10 mm thick)
Storing (cold, hot)
Handling in the field (pad eye, no strap) Drive the piles (splice, temperature)
Clean surfaces (free of dust and grease)
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Clean surfaces (free of dust and grease)
Apply the primer (wait 24 hrs for drying)
Applying the bitumen (10 mm thick)
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Applying the bitumen (10 mm thick)
Applying the bitumen (10 mm thick)
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Applying the bitumen (10 mm thick)
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APPLYING BITUMEN
COATING OUTSIDE INTHE COLD
BITUMEN BECOMES
BRITTLE DUE TO
COATING OUTDOORS
Handling in the field
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Handling in the field
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Storing the coated piles
In hot weather, store under cover away
from direct sunlight
In cold weather, wait until bitumen
cools down before storing
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Driving the piles (splice, temperature)
AVERAGE ANNUAL AIR TEMPERATURE = SOIL TEMPERATURE
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1. Downdrag does not reduce the ultimate capacity
of a pile
2. Downdrag increases the load in the pile
3. Downdrag increases the settlement of a pile;
downdrag is a case where settlement controls the
pile design
4. Live loads should not be included in settlement
calculations including downdrag
General Observations
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6. Downdrag makes a pile come out of the ground
7. Bitumen reduces downdrag significantly (90%?)
when properly chosen and properly applied
8. Bitumen is most easily applied when the air is
colder than the soil (winter) but when the
temperature is above freezing
9. Piles located on the inside of a pile groupexperience less downdrag than the perimeter
piles
General Observations
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F i f i
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Videotape on Bitumen Coating
(email: [email protected])
Briaud J.-L., Tucker L.M., 1998, Design
guidelines for downdrag on uncoated and
bitumen coated piles, NCHRP Report 393,
National Academy of Sciences.
For more information
On Downdrag
https://ceprofs.tamu.edu/briaud/NCHRP_Downdrag.pdf
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THANK YOU
http://ceprofs.civil.tamu.edu/briaud/
http://www.issmge.org/
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Full Scale Field Test:
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Full Scale Field Test:
New Orleans, Louisiana, USA
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Full Scale Field
Test:
New Orleans,Louisiana, USA
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Full Scale Field
Test:
New Orleans,Louisiana, USA
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STEEL WOOD PPC
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APPLYING PRIMER
COATING WITH BITUMEN
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Full Scale Field
Test:
Alberta,Edmonton, Canada